Time-stamped digital and time-stamped analog alarms differ to other alarm types, as they do not rely on the polling of variables to determine alarm conditions. They operate via a process where the Alarm Server is notified of any value changes to a specified variable using the AlarmNotifyVarChange Cicode function.
The Alarm Server uses this information to update alarms that monitor the variable. This process allows an accurate timestamp to be associated with an alarm condition.
This process updates the Var Tag A and Var Tag B properties for time-stamped digital alarms.
Events trends can be used in conjunction with time-stamped digital alarms to provide millisecond accuracy for both trend and alarm data. See TrnEventSetTable and TrnEventSetTableMS Cicode functions.
To configure a time-stamped digital alarm:
Note: For time-stamped digital alarms to function correctly, you need to configure the Cicode function AlarmNotifyVarChange, as it notifies the alarm server of any value changes for the associated variable. It also passes the timestamp with the notification message. The AlarmServer maintains a queue for notification messages and runs a separate task to check this queue.